1. Field of the Invention
The present invention relates to ballasts for powering high pressure gas discharge lamps. It finds application in conjunction with a single ballast for starting/powering a single high pressure gas discharge lamp or a plurality of high pressure gas discharge lamps connected in series and will be described with particular reference thereto. In addition, the present invention will find application where any standard ballast finds application, particularly to gas discharge lamps.
2. Discussion of the Art
A high pressure discharge lamp, such as a metal halide, mercury, or high pressure sodium lamp, is typically powered by an electromagnetic ballast incorporating an iron core. The ballast receives voltage from a power source and outputs a ballast voltage for driving the lamp. The ballast, which uses an iron core and wire to achieve the necessary ballasting functions or current limiting functions represents a major component of ballast cost, as well as bulk.
The foregoing type of ballast typically powers only a single high pressure lamp. There is usually a correlation with lamp performance and the fill pressure of the arc tube. Higher arc tube fill pressures lead to improved lamp lumen depreciation (light output with time) but there is a corresponding increase in the voltage needed to start these lamps. One method that has been suggested to be used to accommodate a higher than normal fill pressure is to use a diode internal to the lamp in series with the starting electrode ballasting resistor. The diode serves to charge the capacitor on lead circuitsxe2x80x94effectively increasing the open circuit voltage available to start the lamp. This method has been described for use as a retrofit for mercury ballasts which power Metal Halide lamps, as described, for example, in U.S. application Ser. No. 09/290,008, entitled Enhanced Lumen Maintenance of Metal Halide Lamps By Increased Cold Gas Fill, filed Apr. 9, 1999, and U.S. application Ser. No. 09/460,177, entitled Active Diode Protection Apparatus In Metal Halide Lamps, filed Dec. 10, 1999, both of which are assigned to the assignee of this application.
The present invention provides a new and improved apparatus and method which overcomes the problems of existing ballasts.
A ballast for a discharge lamp is powered by an input power source and supplies an output load comprising at least one discharge lamp. A power capacitor, during operation, is electrically connected to both the power source and the at least one lamp. A switching circuit is electrically connected to the capacitor and the at least one lamp. A current created by the power source flows through the switching circuit, and by-passes the at least one lamp until a voltage on the capacitor is sufficient to start the lamp. Under normal operation current flows through the switching circuit at least until the charge on the capacitor is sufficient to start the at least one discharge lamp. In certain situations, the voltage may reach a maximum voltage which is not sufficient to start the lamp.
By the foregoing design more voltage is delivered to start the lamps than that delivered in previous designs, and requires less material, is cheaper to manufacture, and has a lower operating cost than previous designs.